Supplementary material from "Improving influenza forecast in the tropics and subtropics: a case study of Hong Kong"

Influenza forecasts could aid public health response as shown for temperate regions, but such efforts are more challenging in the tropics and subtropics due to more irregular influenza activities. Here, we built six forecast approaches for influenza in the (sub)tropics, with six model forms designed to model seasonal infection risk (i.e., seasonality) based on the dependence of virus survival on climate conditions and to flexibly account for immunity waning. We ran the models jointly with the ensemble adjustment Kalman filter to generate retrospective forecasts of influenza incidence in subtropical Hong Kong from January 1999 to December 2019 including the 2009 A(H1N1)pdm09 pandemic. In addition to short-term targets (1- to 4-week ahead predictions), we also tested mid-range (1-3 months) and long-range (4-6 months) forecasts, which could be valuable for long-term planning. The largest improvement came from the inclusion of climate-modulated seasonality modeling, particularly for the mid-range and long-range forecasts. The best-performing approach included a seasonal-trend-based climate-modulation and assumed mixed immunity waning; the forecast accuracies including peak week and intensity were comparable to that reported for temperate regions including the United States. These findings demonstrate that incorporating mechanisms of climate modulation on influenza transmission can substantially improve forecast performance in the (sub)tropics.